The invention relates to an electrochemical cell with an improved insulating gasket resulting in an improved seal. The invention also relates to a process for producing the cell.
The miniaturization of electronic devices has created a demand for small but powerful electrochemical cells. Cells that utilize an alkaline electrolyte are known to provide high energy density per unit volume and are, therefore, well suited for applications in miniature electronic devices such as hearing aids, cameras, watches and calculators. However, alkaline electrolytes, such as aqueous potassium hydroxide and sodium hydroxide solutions, have an affinity for wetting metal surfaces and are known to creep through the sealed metal interface of an electrochemical cell. Leakage in this manner can deplete the electrolyte solution from the cell and can also cause a corrosive deposit on the surface of the cell that detracts from the cell""s appearance and marketability. These corrosive salts may also damage the device in which the cell is housed. Typical cell systems where this problem is encountered include silver oxide-zinc cells, nickel-cadmium cells, air depolarized cells, and alkaline manganese dioxide cells.
In the prior art it has been a conventional practice to incorporate insulating gaskets between the cell""s cup and can so as to provide a seal for the cell. Generally, the gasket must be made of a material inert to the electrolyte contained in the cell and to the cell environment. In addition, it must be flexible and resistant to cold flow under pressure of the seal, and it must maintain these characteristics so as to insure a proper seal during long periods of storage. Materials such as nylon, polypropylene, ethylene-tetrafluoroethylene copolymer and high density polyethylene have been found to be suitable as gasket materials for most applications. Typically, the insulating gasket is in the form of a xe2x80x9cJxe2x80x9d shaped configuration with a xe2x80x9cUxe2x80x9d shaped groove, into which the extended wall of the cup is inserted. When the cup and gasket are inserted into the can and the cell is radially squeezed, a flange of the gasket forms a seal with the bottom portion of the wall of the cup. The gasket generally extends the entire length of the peripheral wall of the cell and generally has a relative flat wall component having a uniform thickness. To better insure a good seal, a sealant is generally applied to the gasket, including its xe2x80x9cUxe2x80x9d shaped groove. Upon insertion of the cup into the gasket, the edge of the extended wall of the cup will seat in the sealant, and then, upon the application of a compressive force, the wall of the gasket will be compressed against the edge of the extended cup wall.
It is desirable to minimize the volume taken up by the gasket of an electrochemical cell in order to maximize the amount of the total internal volume of the cell that is available for active materials. This is particularly true in miniature cells, in which the volume of the gasket is a relatively large percentage of the total cell volume. However, the thinner the gasket wall, the smaller the range of compression of the gasket wall (in inches or millimeters) between the parts of the cell container to be sealed that will result in a good seal. The smaller acceptable dimensional range makes variation in dimensions of the cell parts and variation in the dimensions of the closed cell more critical and more difficult to control. This is further complicated by greater difficulty in molding thinner wall gaskets, leading to increased variation in wall thickness. In addition, it is more difficult to sufficiently compress the wall of thinner gaskets using moderate pressures, while the anode cup of a miniature cell may be deformed if larger forces are applied. As a result, with conventional gaskets there is a practical limit to how thin the gasket wall can be made without sacrificing the effectiveness of the seal.
Another problem that can occur with gaskets of a conventional design is the loss of adequate compressive stress if the adjacent walls of the can and cup spread apart. This may occur shortly after closing the cell and/or over time due to a tendency of the can wall to relax and return to its previous larger diameter. Because thinner gaskets will maintain adequate compressive stress for a good seal over a smaller range of compression (in inches or millimeters) of the gasket wall, the thinner gaskets are more prone to losing adequate compressive stress when the can and cup walls spread apart.
Cold flow, which is the tendency of a plastic to flow at temperatures below its melting point when a compressive force is applied, also contributes to the loss of compressive stress in gaskets over time. The rate of cold flow can increase considerably when cells are exposed to higher temperatures. Stiffer materials are generally more resistant to cold flow, but other sealing properties of these stiffer materials tend to be inferior.
In U.S. Pat. No. 4,656,104, Tucholski discloses a sealing gasket for an electrochemical cell that has a vertical peripheral wall that is tapered and/or stepped so that the average thickness is less at the top than at the bottom. Advantages of this type of gasket include a reduction in the vertical force required to crimp the sidewall of the container and a reduction in the volume of gasket material squeezed between the bent over portion of the container sidewall and the cell cover. Reducing the thickness of the upper portion of the gasket wall also reduces the internal cell volume taken up by the gasket, but not by as much as if the thickness of the entire peripheral wall of the gasket were reduced.
In U.S. Pat. No. 4,258,108, Glaser discloses a method of leak-proofing button cells by placing a hydroxide binding material on the side of the sealing region facing away from the electrolyte. The hydroxide binding material may be positioned on the outer side of the sealing region, or it may be positioned in grooves which encircle the plastic seal. The hydroxide binding material is intended to reduce the escape of electrolyte from the cell due to creepage of the KOH electrolyte. It does not, however, improve the sealing characteristics of the gasket itself, thereby allowing the use of a gasket with a thinner wall or the use of stiffer gasket materials that are more resistant to cold flow.
Stark et al., in U.S. Pat. No. 3,723,184, disclose a battery cell closure comprising a cell container having alternate upstanding projections and depressions along the inner sidewall of its mouth portion and a compressible insulating sealing ring interposed between the top closure of the cell and the cell container. The disclosed invention is intended to prevent longitudinal displacement of the sealing ring upon application of a radial force, such as that effected by crimping the cell, as a result of flowing of sealing ring material into to grooves. Stark et al. do not disclose how their invention might allow the use of a gasket with a thinner peripheral wall, and the required projections and depressions along the inside of the mouth portion of the cell container make manufacture of the container more difficult and expensive.
In U.S. Pat. No. 4,791,034, Dopp discloses the use of a sealing sleeve having inwardly protruding ridges to seal the interface of metal components of electrochemical cells and the like. An improved seal results; however, further improvement and/or reduction in the wall thickness is desirable.
It is an object of the present invention to provide an electrochemical cell with a sealing gasket, having a wall component in which both of its surfaces have at least one compressed projection, with improved sealing characteristics.
It is also an object of the present invention to provide an electrochemical cell with a thinner gasket wall, a larger internal volume available for active materials and an increased discharge capacity.
It is another object of the present invention to provide an electrochemical cell with a sealing gasket which is more resistant to cold flow than conventional gaskets.
It is yet another object of the present invention to provide an electrochemical cell with a sealing gasket that will maintain adequate compressive stress to provide an excellent seal when the distance between the sealing surfaces of the cell container increases.
It is another object of the present invention to provide an electrochemical cell with a sealing gasket that is inexpensive and easy to make and for which dimensional tolerances are less critical than conventional gaskets.
It is a further object of the present invention to provide a process for producing an electrochemical cell having the above advantages.
The foregoing and additional objects of the present invention will become more fully apparent from the following description and drawings.
One aspect of the invention relates to an electrochemical cell comprising:
(a) a first electrode;
(b) a second electrode;
(c) a separator between the first electrode and the second electrode;
(d) an electrolyte;
(e) a two-part conductive housing, the first part being a can with a peripheral wall having a first sealing surface and the second part being a cup with a peripheral wall having a second sealing surface, wherein the first and second surfaces are substantially smooth; and
(f) an insulating gasket comprising a peripheral wall having third and fourth sealing surfaces, wherein:
(i) the peripheral gasket wall is disposed between the peripheral can and cup walls and forms compressive seals between the first surface of the can and the third surface of the gasket and between the second surface of the cup and the fourth surface of the gasket;
(ii) each of the third and fourth surfaces has at least one compressed projection thereon and the at least one compressed projection on the third surface is offset from the at least one compressed projection on the fourth surface in a direction parallel to a longitudinal axis of the peripheral gasket wall; and
(iii) an area of high compressive stress relative to the surrounding area is disposed at each compressed projection.
Another aspect of the invention is an electrochemical cell comprising a two-part conductive housing, the first part being a can with a peripheral wall having a first sealing surface and the second part being a cup with a peripheral wall having a second sealing surface, and an insulating gasket, the gasket comprising a peripheral wall having third and fourth sealing surfaces disposed and compressed between the peripheral can and cup walls; wherein, with the gasket in the normal uncompressed condition:
(a) each of the third and fourth surfaces has at least one projection thereon;
(b) each projection has a center of mass, a point of greatest protrusion relative to the longitudinal centerline and a point of least protrusion relative to the longitudinal centerline;
(c) the center of mass of each projection on the third surface is offset from the center of mass of each projection on the fourth surface in a direction parallel to the longitudinal centerline;
(d) each projection comprises a step having a flat segment, sloping from its point of highest protrusion to its point of lowest protrusion; and
(e) the flat segments of the projections on the third surface are parallel to the flat segments of the projections on the fourth sealing surface.
Yet another aspect of this invention is a process for assembling an electrochemical cell comprising the steps:
a) preparing a conductive can having a peripheral wall with a first sealing surface;
b) preparing a conductive cup having a peripheral wall with a second sealing surface;
c) preparing an electrically insulating gasket having a peripheral wall with third and fourth sealing surfaces, each having at least one projection thereon, wherein the at least one compressed projection on the third surface is offset from the at least one compressed projection on the fourth surface in a direction parallel to a longitudinal axis of the peripheral gasket wall;
d) placing electrode materials, separator and electrolyte within the cup and the can;
e) assembling the can and the cup together so that the sealing surfaces of the peripheral wall of the gasket are in contact with and disposed between the peripheral walls of the can and the cup; and
f) securing the can to the cup so as to compress the at least one projection on both the third and fourth surfaces of the peripheral wall of the gasket, creating an area of high compressive stress at each compressed projection, thereby effectively sealing the can to the cup via the gasket and electrically insulating the can from the cup.